This invention has to do with piezoelectric resonant transducers in which a vibratory diaphragm member is edge mounted and carries a piezoelectric ceramic disk mounted coaxially on at least one of its faces for sensing or driving flexure of the diaphragm.
Flexure of the central portion of the diaphragm member perpendicular to its plane is associated with radial stretching or compression of the ceramic. Those stresses are accompanied by electrical potentials of opposite polarity at the respective disk surfaces, corresponding to the piezoelectric properties of the material. Electrodes connect the opposite faces of the disk with an external circuit of any desired type.
It is well known that such piezoelectric ceramic transducers are useful for sensing and for generating vibratory movement of many different types. In one class of such transducers the diaphragm assembly has a face in direct or indirect contact with a liquid medium or a mechanical member, by which it is constrained against free vibration. Such transducers can interchange energy with the contacted medium or member with moderate efficiency over a broad range of frequencies, which may or may not include the resonant frequency that the diaphragm assembly would have if free. Patent 3,489,995 to J. Laurent describes such a transducer, in which the diaphragm assembly is constrained by direct contact with an elastic cover on one side and by a protective back-up plate closely spaced on the other side.
The present invention is concerned with a different class of piezoelectric transducers in which the diaphragm and its carried disk of ceramic are free to flex in fundamental mode at their natural or resonant frequency of vibration. Under that condition the transducer is a highly sensitive detector of vibrations at that same resonant frequency; and can drive such vibrations with good efficiency in response to an input periodic electrical signal.
In order to insure optimum sensitivity of selective response at a sharply defined target frequency, whether for detection or generation of vibrations, it is important that the resonant frequency of vibration of the transducer match that target frequency as closely as possible, and also that the diaphragm housing structure permit the diaphragm to vibrate freely at its normal resonant frequency.
The utility of such selectively responsive transducers has been limited in the past by difficulty in producing economical transducers having a well-defined resonant frequency that is satisfactorily uniform and stable. Under conditions of mass production, especially when the production cost must be held to a minimum, individual transducers tend to differ in resonant frequency over a range that is excessive for many applications.
In accordance with one aspect of the invention, it has been discovered that much of the variability in frequency between individual transducers is due to small variations in the edge-clamping structure, and in the manner in which that clamping structure engages the periphery of the vibratory diaphragm. Even a small change in effective diameter of the annular clamped region can alter the resonant frequency; and a lack of symmetry in the clamping action, such as may be caused by imperfect assembly, for example, appears to alter the response to vibration by modifying the mode of vibration.
Also, the resonant frequency is ordinarily sensitive to changes in temperature. For some applications, including those in the automotive field, for example, the resonant frequency must remain as uniform as possible over a temperature range of several hundred degrees, for example from about -40.degree. to +300.degree. F. When a transducer is cycled over any such temperature range, the resonant frequency ordinarily varies appreciably, due in large part to stresses developed by differing thermal expansion of its various components.
More particularly, the materials most suitable for clamping the rim of the diaphragm, such as stainless steel, for example, have coefficients of thermal expansion appreciably higher than the materials ordinarily used for the diaphragm member. Under that condition, increasing temperature causes the mounting to expand faster than the clamped diaphragm rim, tending to stress the diaphragm radially outward and thereby increase the resonant frequency.